The present invention enables more precise control of a micro-actuator than in previous micro-actuator systems. For example, the present invention may be advantageously employed to position the read/write heads of a hard disk drive (HDD).
An inductive position sensor for an electromagnetic linear actuator was described in a paper by H. Guckel, T. Earles, J. Klein, D. Zook, and T. Ohnstein entitled "Electromagnetic Linear Actuators with Inductive Position Sensing" appearing in Sensors and Actuators A 53 (1996) at pages 386-391, incorporated herein by reference. The sensor used the self inductance of the coils for position sensing of the plunger. This system employs wire wound coils rather than an integrated coil. Also, the actuator was a solenoid plunger rather than a silicon micromachined device.
Information storage in HDD systems is arranged in concentric "tracks" upon the disks; information density increases when the concentric tracks can be placed closer together. The conventional parameter used to describe this characteristic is "tracks-per-inch" (TPI) which refers to the number of tracks measured along the radius of the disk.
There are several limitations in conventional HDD systems which prevent TPI from increasing much beyond today's state-of-the-art. The actuator arm (FIG. 1) which rotates about its pivot point determines the position of the read/write head. As TPI increases, the size of the rotation angle corresponding to a single track obviously decreases. Friction, ball slippage, and other nonlinear phenomena within the pivot and its bearings, and within flexible electrical wiring or spring mechanisms, makes control to smaller rotation angles very difficult, if not impossible. Furthermore, flexure or mechanical resonances within the actuator arm itself introduce small movements in the position of the read/write head which are essentially independent of the actuator arm's rotational movement within its bearings. Although these small movements are not serious for a low capacity HDD, they are serious limitations if one wishes to increase TPI.
In addition to limitations within the actuator arm, support bearings, and other associated structures, other disturbances also occur due to interactions between the actuator arm, read/write head, and the rotating disk. Windage effects, spindle motor eccentricities, and spindle motor cogging are examples of mechanical "run out" errors which limit practical TPI levels for today's HDD systems.
Another limitation is errors which occur when the "master" tracks are defined for the HDD. Servo write "errors" are imperfections in the concentric track locations and therefore become another limitation which the servo system must accommodate.
If a HDD is fabricated with higher bandwidth servo control, many of the limitations from friction and ball bearings can be overcome. However mechanical resonances of the arm itself not only represent small movements, but they also prevent fabrication of higher bandwidth servo control which would help overcome these other limitations.
If one attempts to increase the bandwidth beyond the limitations imposed by the mechanical resonances, the control system becomes unstable and operation is impossible. Conventional methods for reducing mechanical resonance limitations are to increase the thickness, strength, weight, and manufacturing precision of the actuator arm. Obviously this is counter to the design goals for low power, low cost HDD systems.